Function getHookArgumentsLength

static/vuejs/vue.esm.js:6642–6655

* Normalize a transition hook's argument length. The hook may be: * - a merged hook (invoker) with the original in .fns * - a wrapped component method (check ._length) * - a plain function (.length)

(fn)

 * - a plain function (.length)
 */
function getHookArgumentsLength (fn) {
  if (!fn) { return false }
  var invokerFns = fn.fns;
  if (invokerFns) {
    // invoker
    return getHookArgumentsLength(
      Array.isArray(invokerFns)
        ? invokerFns[0]
        : invokerFns
    )
  } else {
    return (fn._length || fn.length) > 1
  }
}

function _enter (_, vnode) {
  if (!vnode.data.show) {